A top diplomat says the U.S. will soon launch a major border security program to help Nigeria and neighboring countries fight the militant group Boko Haram.

U.S. Assistant Secretary of State for African Affairs Linda Thomas-Greenfield spoke to officials in the Nigerian capital Abuja on Thursday, as Nigerian newspapers reported that the militants have seized the city of Bama and other locations in the northeastern state of Borno.

Thomas-Greenfield said the U.S. is "very troubled by the apparent capture of Bama" and the prospects for an attack on the state capital Maiduguri.

She said "the frequency and scope of Boko Haram's terror attacks have grown more acute and constitute a serious threat" to Nigeria's overall security.

Witnesses reported a major clash between Nigerian soldiers and Boko Haram in Bama, a city of more than 200,000 people, on Monday. The army and local officials said troops drove back the militants' initial attack but that Boko Haram returned with more fighters later in the day. The fighting prompted thousands of local residents to flee to Maiduguri.

Boko Haram has seized control of several towns and cities in Borno since the group's leader, Abubakar Shekau, announced the group was setting up a caliphate, a state under Islamic law.

State radio in neighboring Cameroon said the militants had captured the border town of Banki and that 400 Nigerian soldiers had fled into Cameroon to escape intense fighting.

Nearly 500 Nigerian soldiers crossed over into Cameroon in a separate incident last week.

Boko Haram, whose name means "Western education is a sin," has killed thousands of people since launching an insurgency against the Nigerian government in 2009. The group also kidnapped more than 200 girls from a school in the town of Chibok in April.

Nigeria's government declared a state of emergency in three states last year and deployed thousands of troops to fight the group. However, analysts have said the militants appear to have better weapons and equipment than government security forces.